These teachings relate generally to detectors of thermal energy and, more particularly, relate to uncooled bolometers that are responsive to Infrared Radiation (IR).
Miniature or microminiature bolometers are employed as detector pixel elements in two dimensional arrays of thermal (IR) detectors. The two dimensional array of bolometers converts the IR arriving from a scene of interest into electrical signals that are applied to a readout integrated circuit (ROIC). After amplification and desired signal shaping and processing, the resulting signals can be further processed as desired to provide an image of the scene of interest.
A microbolometer typically includes a polycrystalline semiconductor material, such as Vanadium oxide (VOx) or Titanium oxide, having an electrical resistivity that varies as a function of temperature. An absorber of IR, such as SiN, is provided in intimate contact with the polycrystalline semiconductor material so that its temperature can be changed as the amount of IR arriving from the scene changes. Preferably, the polycrystalline semiconductor/absorber structure is thermally isolated from the underlying ROIC.
Reference with regard to microbolometers and techniques for fabricating same can be had to commonly assigned U.S. Pat. No. 6,144,030, issued Nov. 7, 2000, xe2x80x9cAdvanced Small Pixel High Fill Factor Uncooled Focal Plane Arrayxe2x80x9d, by Michael Ray et al., the disclosure of which is incorporated by reference herein in its entirety.
Another U.S. Patent of interest is U.S. Pat. No. 6,201,243 B1, issued Mar. 13, 2001, to Hubert Jerominek.
In general, as multi-level uncooled bolometer unit cell (pixel) sizes are reduced, and the performance requirements are increased, there arises a need to reduce the thermal mass of the bolometer units cells. One technique for accomplishing this is to reduce the component film thicknesses. However, this has the adverse effect of reducing the absorption of IR in the active detector areas, thereby reducing sensitivity. As the thicknesses of the constituent film layers is made thinner, there is a stronger reliance on a resonant cavity effect.
For example, and referring to FIG. 1 of the above-reference U.S. Pat. No. 6,144,030, there is an optical resonant cavity 22 formed between an IR absorptive structure 12, which includes a VOx semiconductor strip 14, and a thermally isolating structure 20 that includes a planar member 26 that also functions as a reflector.
In U.S. Pat. No. 6,201,243 B1 a mirror 3 is located on the substrate and is spaced apart from a microstructure 22, that contains the VOx thermistor, by one xc2xc wavelength in the center of the IR spectral band of interest. This is said to gain resonant performance.
As can be appreciated, as the film thicknesses are reduced the overall structure tends to become less robust, thereby complicating the manufacture, handling and use of the microbolometer array. Reduced film thicknesses can also make the constituent layers more sensitive to intrinsic stresses, resulting in non-planarity or warping of the layers.
Furthermore, as film thicknesses are reduced, and more reliance is placed on the operation of the resonant optical cavity, it can be appreciated that the cavity construction should be optimized for its intended purpose. However, the placement of meander lines or other structures at a boundary of the cavity can impair its usefulness for its intended purpose.
The foregoing and other problems are overcome by methods and apparatus in accordance with embodiments of these teachings.
A microbolometer unit cell is constructed as a multi-level device having a lower-level thermal isolation structure and an upper-level structure containing an IR absorber/thermistor composite layer. The device further includes a middle-level reflector layer. An optical resonant cavity is formed between the reflector layer and the overlying absorber/thermistor composite layer, and the optical resonant cavity is physically, electrically and optically decoupled from the underlying thermal isolation structure. If desired, a stiffening member can be added to the absorber/thermistor composite layer, preferably in the form of an increased layer thickness at a periphery of the absorber/thermistor composite layer.
It is also within the scope of these teachings to make one subset of unit cells of the set of unit cells sensitive to one wavelength of IR, and to make at least one other subset sensitive to another wavelength of IR, thereby providing a two-color or a multi-color microbolometer array.
These teachings enable both the layer thicknesses and the unit cell center-to-center pitch to be reduced, as compared to prior art designs, thereby decreasing thermal mass and increasing the frequency response, but without degrading sensitivity, as the optical resonant cavity is improved over conventional approaches, and is optimized for its intended purpose.
A microbolometer unit cell includes a substantially planar upper-level incident radiation absorption and detection structure, a substantially planar, preferably stress-balanced, middle-level radiation reflection structure that is spaced apart from the upper-level incident radiation absorption and detection structure for defining an optical resonant cavity there between, and a substantially planar lower-level thermal isolation leg structure spaced apart from the middle-level radiation reflection structure and electrically coupled to the upper-level incident radiation absorption and detection structure and to an underlying readout circuit. The lower-level thermal isolation leg structure is electrically coupled to the upper-level incident radiation absorption and detection structure through a leg that passes through an aperture within the middle-level radiation reflection structure, the leg also functioning as a structural support member. The lower-level thermal isolation leg structure is electrically coupled to the readout circuit through another leg that terminates on an electrical contact disposed on an underlying readout integrated circuit, and the middle-level radiation reflection structure is supported by an extension of this leg. It is within the scope of these teachings that the upper-level incident radiation absorption and detection structure includes a stiffening member, such as one disposed frame-like about a periphery of the upper-level incident radiation absorption and detection structure.
The resonant optical cavity is defined by a spacing that is a function of the wavelength of the incident radiation, and an adjacently disposed unit cell of an array of unit cells may have a resonant optical cavity having a different spacing, thereby providing enhanced sensitivity to a different wavelength.